This invention relates to illumination systems for reflective displays. More specifically, this invention relates to a display and means for projecting patterned light onto at least a part of the display. This invention also relates to a method for directing spatially and spectrally modulated illumination on to a reflective display to improve contrast and colorfulness under ambient lighting conditions. Certain embodiments of the present invention relate to a display, intended primarily for outdoor use but which may also find some application indoors (in the sense of use within buildings, tents and other similar structures); these embodiments of the display of the present invention make use of a reflective bistable electro-optic display in conjunction with a light source arranged to illuminate the reflective electro-optic display.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned, the change in optical state may not be a color change at all. The terms “black” and “white” may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example the aforementioned white and dark blue states. The term “monochrome” may be used hereinafter to denote a drive scheme which only drives pixels to their two extreme optical states with no intervening gray states.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that such electro-wetting displays can be made bistable.
One type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 7,002,728 and 7,679,814;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276 and 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. Nos. D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564; 6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438; 6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519; 6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769; 6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,967,640; 6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296; 7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752; 7,176,880; 7,190,008; 7,206,119; 7,223,672; 7,230,751; 7,256,766; 7,259,744; 7,280,094; 7,327,511; 7,349,148; 7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572; 7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712; 7,583,427; 7,598,173; 7,605,799; 7,636,191; 7,649,674; 7,667,886; 7,672,040; 7,688,497; 7,733,335; 7,785,988; 7,843,626; 7,859,637; 7,893,435; 7,898,717; 7,957,053; 7,986,450; 8,009,344; 8,027,081; 8,049,947; 8,077,141; 8,089,453; 8,208,193; and 8,373,211; and U.S. Patent Applications Publication Nos. 2002/0060321; 2004/0105036; 2005/0122306; 2005/0122563; 2007/0052757; 2007/0097489; 2007/0109219; 2007/0211002; 2009/0122389; 2009/0315044; 2010/0265239; 2011/0026101; 2011/0140744; 2011/0187683; 2011/0187689; 2011/0286082; 2011/0286086; 2011/0292319; 2011/0292493; 2011/0292494; 2011/0297309; 2011/0310459; and 2012/0182599; and International Application Publication No. WO 00/38000; European Patents Nos. 1,099,207 B1 and 1,145,072 B1;        (e) Color formation and color adjustment; see for example U.S. Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155; 7,667,684; 7,791,789; 7,956,841; 8,040,594; 8,054,526; 8,098,418; 8,213,076; and 8,363,299; and U.S. Patent Applications Publication Nos. 2004/0263947; 2007/0109219; 2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970; 2009/0004442; 2009/0225398; 2010/0103502; 2010/0156780; 2011/0164307; 2011/0195629; 2011/0310461; 2012/0008188; 2012/0019898; 2012/0075687; 2012/0081779; 2012/0134009; 2012/0182597; 2012/0212462; 2012/0157269; and 2012/0326957;        (f) Methods for driving displays; see for example U.S. Pat. Nos. 7,012,600 and 7,453,445;        (g) Applications of displays; see for example U.S. Pat. Nos. 6,118,426; 6,473,072; 6,704,133; 6,710,540; 6,738,050; 6,825,829; 7,030,854; 7,119,759; 7,312,784; and 8,009,348; 7,705,824; and 8,064,962; and U.S. Patent Applications Publication Nos. 2002/0090980; 2004/0119681; 2007/0285385; and 2010/0201651; and International Application Publication No. WO 00/36560; and        (h) Non-electrophoretic displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent Application Publication No. 2012/0293858.        
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode. Electro-optic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Other types of electro-optic media may also be used in the displays of the present invention.
Regardless of the exact technology used to display data thereon, electro-optic displays may be grouped functionally into two broad categories, namely emissive displays, in which the light is emitted from or transmitted through the active layer, and reflective displays, in which light is reflected from the active layer. An emissive display conveys information by changes in its luminance and may be viewed in the absence of ambient light, whereas a reflective display conveys information by changes in its reflectance and cannot be viewed without ambient light. Emissive displays may incorporate materials that are intrinsically electroluminescent (for example, organic light-emitting diodes, OLEDs) or may be constructed by combining a transmissive or reflective light modulator with a light source; for example, liquid crystal displays (LCDs) commonly combine a non-emissive light valve layer with a backlight. Digital projectors may be regarded as emissive displays comprising a high-intensity light source and a light modulator plus appropriate lenses to deliver an image to a distant, reflective surface. All emissive displays have the disadvantage that their contrast and colorfulness both depend upon the intensity of ambient light. In very bright environments, such as sunlight, the emitted light may be overwhelmed and the displayed information difficult to see. Reflective displays have the advantage that their contrast and colorfulness are not affected by the level of ambient light, indeed their contrast and colorfulness even improve in very bright light. However, reflective displays are obviously difficult to see in dim light.
A further difficulty arises with reflective displays intended to render colored images. As described, for example, in U.S. Pat. No. 8,054,526, a color filter array may be positioned so that an appropriate black-and-white image is viewed through the color filter array. Although a color image is thus provided to the viewer, the color filters necessarily reduce the amount of light reflected from the display in the white state, and the necessary sharing of the available display surface area between the different color primaries limits colorfulness and color gamut.
Numerous attempts have been made to construct hybrid emissive/reflective displays visible in any ambient light. For example, U.S. Pat. No. 7,170,506 describes a hybrid emissive/reflective display in which the intensity of the emitted light is adjustable. As previously noted, the legibility of information on an emissive display suffers under very bright ambient light, so that the intrinsic luminance of the display must be increased to counter the resultant loss of contrast and colorfulness. Conversely, the legibility of information on a reflective display suffers under very low lighting conditions so that the illumination of the display must be increased accordingly to counter the loss of contrast and colorfulness.
Technical solutions have been developed to increase the contrast ratio of emissive displays, so such displays can display high dynamic range (HDR) still and moving images. Some of these so-called HDR displays combine an LCD color panel with a bright backlight, and modulate this backlight with image information, usually in the form of a low-pass filtered luminance channel (this approach is sometimes termed “local dimming”). In more general terms, HDR display can be accomplished by combining two low dynamic range (or low contrast ratio) devices, and by generating from the HDR image two low dynamic range images for each of the display devices. Examples of modulated backlights include the digital projectors or LED arrays described in U.S. Patent Applications Publication Nos. 2008/0137990 and 2008/0137976. However, no equivalent solution is currently known to increase the contrast and colorfulness of reflective displays.
It is known that spatially congruent combinations of projected and reflective images can have contrast ratios exceeding that of either the separate projected and reflective images. A solution that aims at increasing the contrast of projected images is described in U.S. Pat. No. 6,853,486 (the '486 patent). This patent describes an active projection screen that is in registration with the projected image and is used to increase the contrast of the projected image under bright ambient light that would otherwise diminish the contrast of the same image projected on to a uniformly reflective screen. The main drawback to this system is the need to maintain exact registration between the active projection screen and the projector. Without rigid mechanical coupling of projector and screen the required exact alignment and registration are very difficult to achieve and maintain, and the solution described in the '486 patent, namely electronic coupling between projector and screen via ‘reflectance processor’ and ‘display controller’, is expensive.
The system described in the '486 patent is a large-size emissive (projection) display, and the patent does not disclose any device in which color projection means image and means for rendering a reflective image are incorporated into the same device, such that both images can be rendered in registration with each other. In one aspect, the present invention seeks to increase the contrast and colorfulness of reflective displays, especially hand-held devices, under a wide range of ambient lighting conditions, by combining projected and reflective images.
One specific application where bistable electro-optic displays may be useful is outdoor signs, especially traffic control devices. Historically, traffic light and other traffic control signals have been relied upon incandescent bulbs to generate light; more recently, light emitting diodes (LED's) have begun to be used for this purpose. Both incandescent bulbs and LED's (and indeed, all other emissive light sources) require a continuous source of power, typically mains alternating current power, so that any disruption of the power supply due to equipment failure, weather conditions or traffic accidents will result in failure of the traffic lights, traffic hazards and major disruption of traffic flow. Conventional traffic signals have other disadvantages, including:                (a) false signals can occur as a result of sun light and solar glare; conventional traffic lights must overcome ambient light conditions, including specular reflections from various surfaces of the sign, which can make it difficult to discriminate between off and on states of a particular sign, to be noticeable and effective; even the common use of light baffles and high powered LED or incandescent lamps in the range of 25-100 W do not entirely overcome such problems;        (b) traffic lights are located outdoors and hence are subject to harsh mechanical and environmental conditions; they must withstand mechanical damage and remain operational despite vandalism, mechanical shock and impact, extreme temperatures, and exposure to ultraviolet radiation;        (c) total cost of ownership, particularly operating costs, is very important factor in traffic light usage; in New York city alone there are 11,871 traffic lights, and substantial effort is devoted to reducing power usage, including converting incandescent signals to LED's;        (d) strict weight restrictions exist for streetlights to prevent overloading of signage support structures, so industrial design and weight allocations must be carefully managed for signage; and        (e) the need (in some cases) for increasing traffic signal size may compromise performance in terms of power consumption, weight, and cost.        
Accordingly, the present invention seeks to provide a form of information display (which may have the form of a traffic light or other traffic control device) which overcomes the aforementioned problems of prior art devices.
Similar problems are encountered when the “traffic control device” is an indicator on an automobile or other vehicle. Even relatively high powered bulbs do not guarantee sufficient visibility. For example at highway speeds the driver of one automobile may have to react to a change in the brightness of the brake light of a preceding in a fraction of a second if a crash is to be avoided. Most brake lights on cars have a parabolic reflective enclosure to concentrate the light coming from the bulb. Although such a parabolic reflector does help to concentrate the light from the brake light into a narrow beam, it also concentrates any light incident on the reflective enclosure (for example, from sunlight or light from the headlights of a following vehicle) back through the colored plastic cover, thus creating background reflection that tends to obscure the state of the brake light. In the worst case, when the sun faces the back of the car, the specular reflection of the parabolic reflective enclosure, the reflection of the glossy paint on the car and the reflection from the rear windscreen combine to significantly lower the visibility of the brake light.
In another aspect this invention provides a hybrid emissive-reflective display which can improve the visibility of vehicle-mounted signs in high ambient light conditions, thus providing an added margin of safety and offering the possibility of lowering power consumption.